Electronics assemblies, such as for example inverters, often contain electronic semiconductor switches. The switching operations of the semiconductor switches cause undesired interference signals, which spread in a line-bound form or in the form of electromagnetic radiation.
Among the factors responsible for interference signals that enter the power system supplying the electronics assembly are unavoidable stray capacitances, which the electronics assembly or a unit connected to the electronics assembly has with respect to ground or with respect to a heat sink, which during operation is generally at ground potential. Such stray capacitances may exist for example between ground and internal electrical connecting lines of the electronics assembly, between ground and an electrical connecting lead by which an electrical load (for example a motor) is connected to the electronics assembly, between ground and a component part of the load, and so on. If a DC link is present, it can likewise contribute to the stray inductance.
FIG. 1 shows a circuit of an electronics assembly 100 formed as an inverter. The electronics assembly 100 includes two semiconductor switches S1 and S2, the load paths of which are connected in series with a half-bridge. The half-bridge is supplied with a supply voltage by a voltage source 900, for example a rectifier bridge. An electrical load L is connected by way of a connecting lead 906 to an output 103 (also referred to as phase output Ph) of the electronics assembly 100, which at the same time represents an output of the half-bridge.
The electronics assembly 100 has various assembly-internal stray capacitances C1-C3, and also various assembly-internal stray inductances L1, L2 and LZ.
Further stray capacitances C4 (connecting lead 906) and C5 (load) are assembly-external stray capacitances.
C1 is the output stray capacitance of the electronics assembly, C2 is the stray capacitance of the connecting line 901 for distributing the positive supply potential DC+, and C3 is the stray capacitance of the connecting line 902 for distributing the negative supply potential DC−.
L1 is the stray inductance of the connecting line 901 for distributing the positive supply potential DC+, and L2 is the stray inductance of the connecting line 902 for distributing the negative supply potential DC−, and LZ is the stray inductance of a DC link capacitor.
Altogether, the stray capacitances are responsible for so-called common-mode interferences, which produce interferences on the ground line of the system. Differential interferences are caused primarily by the stray inductances and couple into the phases of the system.
It is usually attempted to keep the stray capacitance C1 at the output 103 of the half-bridge, and consequently the interferences caused by it, as small as possible. However, the reduction in the interferences that is brought about by this measure is often insignificant, since in practice the sum of the external stray capacitances C4 and C5 is often much greater than C1.
Furthermore, to reduce interferences, it is often attempted to choose the capacitances C2 and C3 to be as equal in magnitude as possible, which usually requires a special, complex construction of the electronics assembly.
In order to reduce the interferences caused by the stray capacitance C4+C5, which in comparison with C1 is usually very much external to the module, interference-suppression capacitors C+ and C− are often provided between ground and the connecting line 901 for distributing the positive supply potential DC+ and between ground and the connecting line 902 for distributing the negative supply potential DC−. With the aid of the interference-suppression capacitors C+, C−, the interference circuits within the electronics assembly are shorted with respect to the heat sink (which is at ground), so that the interfering currents flowing via the system ground are prevented, or at least reduced significantly. However, these interference-suppression capacitors C+, C− are in turn accompanied by internal stray inductances L+ and L−, which makes the interference emissions of the assembly worse. Furthermore, interference-suppression capacitors are conventionally affected by significant additional stray inductance in the connecting lines to ground and to DC+ and DC−.